1. Field of Invention
This invention relates generally to the field of determination of position, time, and attitude by satellite navigation. More particularly, this invention relates to the basic operation of a single-antenna satellite receiver, such as a GPS receiver, to enable it to provide a user with position, time, and three-axis attitude information.
2. Description of Related Art
Ground-based navigation aide, such as VOR (VHF Omni-Range), and NDB (Non-Directional Beacons), as well as navigation by man-made Earth-orbiting satellites are known in the art. Conventional satellite navigation systems comprise three parts: a space segment consisting of Earth-orbiting satellites, a ground segment including a satellite tracking and control network, and a user segment including satellite receivers that users carry to receive the satellite signals. The satellite receivers contain proprietary circuitry and software that process the satellite signals and extract navigation information, such as the user""s position, time, velocity, and attitude.
There are several satellite navigation systems in use today, such as the U.S. GPS (Global Positioning System) and the Soviet GLONASS system. There are also numerous new systems being planned, such as Europe""s Galileo system, the Japanese MTSAT, and the European Geo-stationary Overlay System (EGNOS). In addition, there are planned augmentations of the existing systems in order to provide additional system capabilities. Examples of augmentations of the GPS are the FAA""s (Federal Aviation Administration""s) Wide Area Augmentation System (WAAS), and the FAA""s Local Area Augmentation System (LAAS). The WAAS and LAAS augmentations are specifically intended for improving system accuracy and integrity for aviation applications. The industry for manufacturing GPS receivers is forecast to increase sharply in coming years.
Satellite navigation receivers provide the user with position, time, and velocity information. Additionally, three-axis attitude information is needed (e.g., specified in terms of three Euler angles) for guidance in numerous systems, such as unmanned aerial vehicles (UAV), missiles, aircraft, satellites, and for pointing communications antennas.
The prior art includes various (GPS) satellite receivers capable of delivering attitude information to a user. However, these receivers require signals from two or more well-separated antennas. When space requirements do not permit multiple well-separated antennas, a method using a single antenna is needed.
Furthermore, a single-antenna attitude determination system is preferable because it is potentially less expensive to manufacture and to install. A multiple antenna system with widely separated antennas must be wired together.
Another example where single-antenna attitude determination is beneficial is in general aviation aircraft. At present, the fleet of aircraft is in transition from ground-based navigation methods to satellite-based (mostly GPS) navigation. GPS receivers are being installed in many aircraft. A pilot needs attitude information whenever aircraft are flying in instrument meteorological conditions (IMC). Satellite attitude determination can be used as a primary attitude system, or alternatively, as an emergency backup when the primary attitude system fails. Since general aviation aircraft are currently in transition to navigation by GPS, many of these aircraft already have single-antenna GPS receivers. Therefore, it would be desirable to be able to extract attitude information from GPS data in existing GPS receivers. A single antenna receiver using a dipole antenna to determine attitude information is known. However, it restricts the kind of receiver antenna and can only determine two angles (out of three) of attitude. Dipole antennas cannot be miniaturized because they must be long to provide adequate sensitivity.
Full 3-axis attitude information about a platform contains compass heading information. Consequently, another application of attitude determination is to replace the wet compass. A miniaturized single-antenna system of attitude determination can be carried on a person. Therefore, a miniaturized attitude determination system has applications for land-based navigation for foot soldiers and civilians.
Prior art satellite navigation methods provide attitude information of the user platform (in addition to position and time), by utilizing two or more antennas to receive satellite signals. The difference in phase of the received signals (by the two or more antennas) is related to the range differences from the satellite to each antenna. The antennas must be widely separated, such as on the wing tips of an aircraft, in order to measure a phase difference associated with the time of flight of the signal from satellite to each of the receiving antenna. This technique is described and reviewed in detail in C. E. Cohen, xe2x80x9cAttitude Determination, Chapter 19, in Global Positioning System: Theory and Applications vol. IIxe2x80x9d; B. W. Parkinson and J. J. Spilker, eds., xe2x80x9cProgress in Astronautics and Aeronautics,xe2x80x9d vol. 163 and 164, American Institute of Aeronautics and Astronautics, Washington, D.C., 1996. Both of these references are incorporated by reference herein in their entireties. Essentially all satellite navigation receivers to date use this method, based on multiple antennas. Examples of GPS receivers that operate on the multiple antenna method of attitude determination described above are: the TANS Vector GPS receiver by Trimble Co. Inc., and the SS Loral Tensor GPS receiver. See also the web pages:
http://horse.mes.titech.ac.jp/srtlssp/HIEN/ENGLISH/system/gpsAtti.html, http://www.ee.surrey.ac.uk/SSC/SSHP/list/list_gps.html and http://www.esa.int/est/prod/prod0070.htm.
In addition to the above multi-antenna techniques of attitude determination mentioned above, Alfred Krall (ASEE) and the present inventor, Thomas B. Bahder (ARL), have filed three patent disclosures with the Army Research Laboratory legal office. These patent disclosures describe a single-antenna satellite method to determine receiver platform attitude (in addition to position and time). However, these three patent disclosures treat a specific, case, where the receiver antenna is a short dipole. A dipole-receiving antenna is rather unrealistic, since, in actual current application of satellite navigation, more compact (smaller size) and complex antennas are used. In addition, the algorithms of Krall and Bahder cannot be trivially extended to deal with more realistic antennas that currently are or would actually be used in practice for satellite navigation.
Furthermore, in these three patent disclosures by Krall and Bahder, the algorithms permit only a partial determination of attitude. Specifically, the algorithms of Krall and Bahder only permit determining two attitude angles, wherein a complete specification of attitude requires that three angles be determined. Their patent disclosures and Journal of Applied Physics article that describes their algorithm of attitude determination, is in xe2x80x9cOrientation and velocity effects in the Global Positioning Systemxe2x80x9d by Alfred B. Krall and Thomas B. Bahder, published in Journal of Applied Physics, vol. 90, No. 12, p.6513 (2001).
The present invention has three primary advantages over the prior art: it uses signals from a single antenna, an arbitrary type of receiving antenna can be used, even a short dipole, it allows miniaturization of the receiver, and all three Euler angles of attitude can be determined. A detailed technical description of this method has been has been described in an article submitted to Journal of Applied Physics, entitled xe2x80x9cAttitude determination from single-antenna carrier phase measurementsxe2x80x9d by Thomas B. Bahder. (This article is scheduled for publication in Journal of Applied Physics in March or April 2002.)
Accordingly, the present invention is directed to single-antenna determination of position, time, and attitude by satellite navigation using a satellite receiver, such as a GPS receiver, which is capable of providing a user with position, time, and three-axis attitude information, while using signals from a single antenna.
The present invention determines position, time, and three degrees of freedom of attitude, from electromagnetic signals received by distant receivers. The transmitters can be on the ground, or satellites that orbit the Earth, such as GPS satellites. The present invention requires the user to have only a single antenna to track the satellite signals. The technique requires that the transmitters (e.g., satellites) are far from the user (observer), so the user""s receiver is in the far-field region of the electromagnetic field of the navigation signals generated by the transmitters.
The present invention is applicable to all types of antennas for receiver and transmitter. It is based on comparing the phase of an open-circuit voltage in a receiver antenna, to a reference phase of a local oscillator in the receiver. The phase difference, between the open-circuit antenna voltage and the local oscillator, depends on the range to the transmitter and on the attitude of the receiving antenna with respect to the transmitter antenna. If the positions and attitudes of the transmitting antennas are known, then a user who is receiving signals from several transmitters can compute his position and attitude with respect to the transmitting antennas. When the positions and attitudes of the transmitting antennas are known in a coordinate system, e.g., the Earth Centered Inertial Frame (ECI), the receiver can determine its position, time and attitude in the same frame. Knowing the time, the receiver can then determine its attitude with respect to a topocentric coordinate system using a clock.
The transmitting antennas can be on navigation satellites, such as GPS, GLONOSS, Galileo, MTSAT or EGNOS. When position, time, and attitude must be determined, the minimum number of necessary signals is seven. This number of signals can be reduced to six, when for example, the oscillator is sufficiently accurate with respect to coordinate time of the navigation satellites, or one of the three Euler angles of the attitude to be determined is set to a constant, or when additional navigation systems (such as inertial navigation) are used to supply partial navigation information, such as one Euler angle of attitude. The signals can be adjusted to take into account a small effect from aberration of starlight, atmospheric time delay due to effective index of refraction of the atmosphere and Faraday rotation induced in the polarization vector due to the magnetic field of the Earth. The transmitting antennas can be replaced by a simulation system. This method of single-antenna attitude determination is also applicable as a basis for the construction of satellite simulators that will faithfully reproduce (simulate the effect on a received signal) the effects of attitude changes (in addition to position and time).